Automobile paint film-protective sheet

ABSTRACT

An automobile paint film-protective sheet comprising a substrate having formed on one side thereof a rubber-based pressure-sensitive adhesive having a dynamic modulus of from 2×10 5  to 7×10 6  dyne/cm 2  at 60° C.

This is a Continuation of application Ser. No. 08/368,401 filed Jan. 3,1995, now U.S. Pat. No. 5,747,132 which is a continuation of applicationSer. No. 07/900,293 filed Jun. 18, 1992, now abandoned.

FIELD OF THE INVENTION

The present invention relates to an automobile paint film-protectivesheet which is hard to cause a paint film deformation and suitable foruse in the surface protection of automotive bodies and other parts.

BACKGROUND OF THE INVENTION

An effective measure is desired to prevent automobile paint films fromsuffering damages, dulling, discoloration, and other defects due tovarious suspended or impinging matters such as dust particles andraindrops. Such paint film protection is necessary, for example, in thecase of loading painted automobiles or automotive parts into trucks orships and transporting these to remote places such as overseas.

A method of covering painted automobiles or automotive parts with a 5-20μm-thick coating comprising a wax has been known as a measure for suchpaint film protection. However, this method has had, for example, thefollowing various problems. It is difficult to form a wax coating at auniform thickness, so that uniform protection of the paint film cannotbe obtained; wax coatings are prone to be fouled and have poorresistance to acid rain; part of the wax may penetrate into the paintfilm to cause discoloration or other defects; formation of a wax coatingand removal thereof require much labor; and use of solvents andtreatment of waste liquids are apt to lead to environmental problems.

On the other hand, various kinds of surface-protective sheets are knownwhich comprise a substrate and a pressure-sensitive adhesive layerformed thereon. Of these, a protective sheet comprising aradiation-curable pressure-sensitive adhesive layer having a loweredglass transition point has been proposed for use on adherends having apaint film (JP-A-2-199184). (The term "JP-A" as used herein means an"unexamined published Japanese patent application".)

The above surface-protective sheet, however, has been defective in thatpreparation of a radiation-curable pressure-sensitive adhesive used toform the pressure-sensitive adhesive layer requires a complicated andspecial technique. In addition, the protective sheet has had otherproblems that the pressure-sensitive adhesive layer obtained shows pooradhesion strength and that the pressure-sensitive adhesive layer haspoor heat resistance, probably because the adhesive layer has been madeto have a lowered glass transition point to impart a particularperformance required in paint film-protective sheets, i.e., preventingthe deformation of the paint film. Due to such a pressure-sensitiveadhesive layer, application of the protective sheet to automobiles hashad a problem that the adhesion strength of the sheet applied to anautomobile increases with increasing temperature during outdoortransportation, etc., making stripping of the sheet difficult. Thus, theproposed surface-protective sheet has been unsatisfactory in basicperformance.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an automobile paintfilm-protective sheet which has an excellent ability to maintain theinitial adhesion strength thereof even when exposed totemperature-rising conditions due to, for example, outdoortransportation of automobiles covered with the protective sheet, whichretains an excellent balance between protective properties due to goodadhesion and strippability over a long period of time, and which is hardto cause the paint film covered with the sheet to suffer a deformation,such as formation of a gap as illustrated in the FIGURE. The FIGUREillustrates minute deformations 21 formed in an automobile paint film 2near the boundaries between the area to which a paint film-protectivesheet 1 comprising a substrate 11 and a pressure-sensitive adhesivelayer 12 is bonded and the area not covered with the protective sheet 1.Such minute deformations can usually appear when a paint film-protectivesheet is applied to a paint film and allowed to stand for about 10 hoursat a temperature slightly lower than the heat distortion temperature ofthe paint film, and the depth (d) of gaps such as adepression-projection deformation generally is about from 0.1 to 0.5 μm.

The present invention provides an automobile paint film-protective sheetcomprising a substrate having formed on one side thereof a rubber-basedpressure-sensitive adhesive having a dynamic modulus of from 2×10⁵ to7×10⁶ dyne/cm² at 60° C.

The present invention further provides an automobile paintfilm-protective sheet comprising a substrate having formed on one sidethereof a pressure-sensitive adhesive layer which is obtained byblending either from 0.005 to 0.5 part by weight of a silicone oil orfrom 0.01 to 2 parts by weight of an acrylic polymer having aweight-average molecular weight of from 1,000 to 50,000 with 100 partsby weight of a rubber-based pressure-sensitive adhesive.

According to the present invention, by using a rubber-basedpressure-sensitive adhesive having a dynamic modulus of from 2×10⁵ to7×10⁶ dyne/cm² at 60° C., the paint film-protective sheet not only ishard to suffer a change in adhesion strength due to the good heatresistance of the pressure-sensitive adhesive layer even undertemperature-rising conditions during outdoor transportation, etc., butalso have excellent two basic performances of protective properties dueto good adhesion and strippability and retain a good balance between thetwo basic performances over a long period of time. Further, by blendinga silicone oil or an acrylic polymer having a molecular weight in thespecific range with a rubber-based pressure-sensitive adhesive, theadhesion strength of the pressure-sensitive adhesive to paint films canbe reduced and regulated in a proper range. Due to this and because ofthe good heat resistance of the rubber-based pressure-sensitive adhesiveused, the adhesion strength of the protective sheet applied to paintedautomobiles or automotive parts is effectively prevented from increasingeven when the protective sheet is exposed to temperature-risingconditions due to outdoor transportation or by other factors. As aresult, adhesive remaining and paint film deformations such as gapformation, particularly at the edge parts of the protective sheet, canbe avoided; such troubles and defects have been involved by the use ofconventional protective sheets due to differences in expansion andcontraction degree between the paint film and the protective sheet withchanging temperature.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a sectional view showing minute deformations formed in anautomobile paint film covered with a conventional paint film-protectivesheet.

DETAILED DESCRIPTION OF THE INVENTION

The automobile paint film-protective sheet of the present inventioncomprises a substrate having formed on one side thereof a rubber-basedpressure-sensitive adhesive layer which may contain a silicone oil or alow molecular weight acrylic polymer, if required and necessary. Theadhesion strength of the protective sheet to automobile paint filmspreferably is from 100 to 1,000 g/20 mm, and preferably from 130 to 700g/20 mm. If the protective sheet has an adhesion strength outside theabove range, there are cases where the protective sheet applied on anautomobile peels off the adherend during transportation of theautomobile or the protective sheet cannot be stripped smoothly aftertransportation of the sheet-covered automobile.

The rubber-based pressure-sensitive adhesive used in the presentinvention needs not be of a curable type. It is, however, preferred thatthe pressure-sensitive adhesive has a dynamic modulus of from 10⁵ to 10⁷dyne/cm² at 60° C. from the standpoint of preventing adhesive remainingor paint film deformations such as gap formation, from occurring at,theedge parts of the protective sheet due to its changes such as expansionand contraction with changing temperature. In particular, in the case ofneither a silicone oil nor a low molecular weight acrylic polymer isblended, it is necessary for the rubber-based pressure-sensitiveadhesive to have a dynamic modulus of from 2×10⁵ to 7×10⁶ dyne/cm² at60° C.

Preferred examples of the rubber-based pressure-sensitive adhesiveinclude those comprising a rubber-type polymer such as natural rubber,polyisobutylene, an A--B--A type block polymer, or the like, andoptionally a tackifier or a softener which imparts the desired dynamicmodulus. The rubber-based pressure-sensitive adhesive may containsuitable fillers and additives such as a pigment, antioxidant, andstabilizer.

From the standpoints of preventing the adhesive strength from increasingwith the lapse of time and avoiding adhesive remaining, minutedeformations of the paint film, etc., it is particularly preferred touse a rubber-based pressure-sensitive adhesive having an solubilityparameter (SP) value. which is different from that of the paint film by1 or more, especially 1.5 or more. From the standpoint of stablymaintaining the adhesive performance over a long period of time,preferred examples of the rubber-type polymer include isobutylene-basedpolymers which do not have unsaturated bond and are not curable, andA--B--A type block copolymers comprising a polystyrene block, anethylene-butylene copolymer block, and a polystyrene block.

As the tackifier and softener, suitable materials may be used, butmaterials having good compatibility with the rubber-type polymer arepreferred. Generally employed tackifiers include hydrocarbon resins,alkylphenol resins, and terpene resins. Generally employed softenersinclude low molecular weight polyisobutylene, in the case where therubber-type polymer is polyisobutylene, and paraffin oils, in the casewhere the rubber-type polymer is an A--B--A type block polymer.

If required and necessary, a silicone oil or a low molecular weightacrylic polymer is added to the rubber-based pressure-sensitive adhesivefor the purpose of reducing the adhesion strength of thepressure-sensitive adhesive and preventing the adhesion strength fromincreasing with increasing temperature thereby to make the protectivesheet hard to cause a paint film deformation such as gaps. Such adhesivestrength control by the addition of a silicone oil or a low molecularweight acrylic polymer is considered to be attributable to migration ofthe silicone oil or acrylic polymer, both of which show poor adhesionand are thermally inactive, to the surface of the pressure-sensitiveadhesive layer (the interface between the pressure-sensitive adhesivelayer and the paint film) due to, for example, the poor compatibility ofthe silicone oil or acrylic polymer with the rubber-basedpressure-sensitive adhesive.

Examples of the silicone oil include dimethylpolysiloxane of thefollowing formula I!, methylphenylpolysiloxane of the following formulaII!, methylhydrogenpolysiloxane of the following formula III!, or thelike. Further, modified silicone oils obtained by incorporating variousfunctional groups into these polysiloxanes to improve solubility inwater, compatibility, reactivity, or other properties can also be used.Examples thereof include epoxy-modified silicone oils, alkyl-modifiedsilicone oils, amino-modified silicone oils, carboxyl-modified siliconeoils, alcohol-modified silicone oils, fluorine-modified silicone oils,alkyl-aralkyl-polyether-modified silicone oils, epoxy-polyether-modifiedsilicone oils, and polyether-modified silicone oils. ##STR1##

Specific examples of the above-described silicone oil include thefollowing materials which all are trade names: SH-200, SH-203, SH-340,SH-3746, SF-8411, SF-8417, SF-8418, SF-8419, SF-8421, SF-8427, FS-1265(manufactured by Toray Silicone Co., Ltd., Japan), TSF-400, TSF-401,TSF-4300, TSF-4445, TSF-4446, TSF-4452, TSF-4460, TSF-4700 (manufacturedby Toshiba Silicone Co., Ltd., Japan), KP-301, KP-310, KP-316, KP-321,KP-322, KP-330, KP-354, and KP-390 (manufactured by Shin-Etsu ChemicalCo., Ltd., Japan).

The amount of the silicone oil added is from 0.005 to 0.5 part byweight, preferably 0.01 to 0.2 parts by weight, per 100 parts by weightof the rubber-based pressure-sensitive adhesive. If the amount of thesilicone oil added is below 0.005 part by weight, a sufficient effectcannot be produced and the adhesion strength of the pressure-sensitiveadhesive is not lowered satisfactorily. If the amount thereof exceeds0.5 part by weight, there are cases where the adhesion strength of thepressure-sensitive adhesive is reduced excessively.

On the other hand, the acrylic polymer used in the present invention isa polymer having a weight-average molecular weight of from 1,000 to50,000. Examples thereof include liquid acrylic polymers having astructural unit represented by the following formula: ##STR2## whereinR¹ is hydrogen or methyl and R² is an alkyl group having from 1 to 18carbon atoms.

Such acrylic polymers can be produced by, for example, polymerizing oneor more alkyl esters of acrylic or methacrylic acid having from 1 to 18carbon atoms in the alkyl moiety, such as ethyl acrylate, n-, i-, ort-butyl acrylate, 2-ethylhexyl acrylate, and 2-ethylhexyl methacrylate,and if required and necessary, other monomer(s), by solutionpolymerization.

In the present invention, a liquid acrylic polymer having a glasstransition point of 0° C. or less is preferably used, which is obtainedusing, as the major monomer component, a monomer that forms a softpolymer having a glass transition point of from -20° to -65° C. Specificexamples of the liquid acrylic polymer include commercially availableproducts such as Modaflow (manufactured by Monsanto Company), PolyflowS, Polyflow No. 9 (both manufactured by Kyoeisha Chemical Co., Ltd.,Japan), and Dispalon #1970 (manufactured by Kusumoto Chemicals Ltd.,Japan).

The amount of the acrylic polymer added is from 0.01 to 2 parts byweight per 100 parts by weight of the rubber-based pressure-sensitiveadhesive. If the amount of the acrylic polymer added is below 0.01 partby weight, a sufficient effect cannot be produced and the adhesionstrength of the pressure-sensitive adhesive is not loweredsatisfactorily. If the amount thereof exceeds 2 parts by weight, thereare cases where the adhesion strength of the pressure-sensitive adhesiveis reduced excessively.

The protective sheet of the present invention can be prepared by theconventional method such as a method in which a solution of thepressure-sensitive adhesive in a solvent or a melt of thepressure-sensitive adhesive is coated on a substrate, or a method inwhich a pressure-sensitive adhesive layer formed on a separator istransferred onto a substrate. The thickness of the pressure-sensitiveadhesive layer to be formed can be suitably determined. Generally,however, the thickness is 200 μm or less, and preferably from 5 to 50μm. If required and necessary, the pressure-sensitive adhesive layerformed on a substrate may be protected by temporarily covering with aseparator, etc., until the protective sheet is subjected to practicaluse.

As the substrate, a suitable material can be used. In general, a plasticfilm, porous film, paper, nonwoven fabric, or the like is used. Thesubstrate generally has a thickness of 300 μm or less, and preferablyfrom 10 to 100 μm. However, the thickness of the substrate is notlimited thereto.

The automobile paint film-protective sheet of the present invention issuitable for use in, for example, the surface protection of automotivebodies or parts covered with a paint film of the melamine-alkyd type,melamine-acrylic type, urethane type or the like, against impingement ofminute particles to the adherends and against chemicals and othersubstances. In particular, the protective sheet can be advantageouslyapplied to automotive bodies or parts which are exposed totemperature-rising conditions due to outdoor transportation or otherfactors or which are surface-protected over a long period of time.

As described above, the automobile paint film-protective sheet accordingto the present invention is hard to cause the paint film to sufferdeformations such as gaps and is also hard to cause adhesive remainingwhen the protective sheet is stripped after use. Furthermore, theprotective sheet has an excellent ability to maintain its initialadhesion strength even under temperature-rising conditions and retainsan excellent balance between protective properties over a long period oftime due to good adhesion and strippability.

The present invention will be explained below in more detail byreference to the following examples, but the invention is not construedas being limited thereto. Unless otherwise indicated, all percents,parts, ratios and the like are by weight.

EXAMPLE 1

A toluene solution of a rubber-based pressure-sensitive adhesivecomprising a mixture of 50 parts of a polyisobutylene having aviscosity-average molecular weight of 1,200,000 and 50 parts of apolyisobutylene having a viscosity-average molecular weight of 35,000was coated on a 40 μm-thick film made of a polypropylene/polyethyleneblend (weight ratio; 1/9), and then dried at 120° C. for 3 minutes,thereby obtaining a protective sheet having a pressure-sensitiveadhesive layer having a dynamic modulus of 1.2×10⁶ dyne/cm² at 60° C.and a thickness of 10 μm. The dynamic modulus was determined by ameasurement with a Vibron tester at a frequency of 1 Hz (hereinafter thesame)

EXAMPLE 2

A protective sheet was obtained in the same manner as in Example 1except that a rubber-based pressure-sensitive adhesive comprising amixture of 25 parts of a polyisobutylene having a viscosity-averagemolecular weight of 1,200,000 and 75 parts of a polyisobutylene having aviscosity-average molecular weight of 35,000 was used to form apressure-sensitive adhesive layer having a dynamic modulus of 4×10⁵dyne/cm² at 60° C.

EXAMPLE 3

A protective sheet was obtained in the same manner as in Example 1except that a rubber-based pressure-sensitive adhesive consisting ofonly a polyisobutylene having a viscosity-average molecular weight of1,200,000 was used to form a pressure-sensitive adhesive layer having adynamic modulus of 3×10⁶ dyne/cm² at 60° C.

EXAMPLE 4

A protective sheet was obtained in the same manner as in Example 1except that a rubber-based pressure-sensitive adhesive comprising amixture of 75 parts of a polyisobutylene having a viscosity-averagemolecular weight of 1,200,000 and 25 parts of a polyisobutylene having aviscosity-average molecular weight of 35,000 was used to form apressure-sensitive adhesive layer having a dynamic modulus of 2.8×10⁶dyne/cm² at 60° C.

EXAMPLE 5

A protective sheet was obtained in the same manner as in Example 1except that a rubber-based pressure-sensitive adhesive comprising amixture of 100 parts of an A--B--A type block polymer comprising apolystyrene block, an ethylene-butylene copolymer block, and apolystyrene block (Kraton G-1657, manufactured by Shell Kagaku K.K.,Japan), and 40 parts of a hydrogenated petroleum resin (Arcon P-100,manufactured by Arakawa Chemical Industries, Ltd.; hereinafter the same)was used to form a pressure-sensitive adhesive layer having a dynamicmodulus of 5×10⁶ dyne/cm² at 60° C.

EXAMPLE 6

A protective sheet was obtained in the same manner as in Example 1except that a rubber-based pressure-sensitive adhesive comprising amixture of 100 parts of an A--B--A type block polymer comprising of apolystyrene block, an ethylene-butylene copolymer block, and apolystyrene block (Kraton G-1652), and 40 parts of the hydrogenatedpetroleum resin was used to form a pressure-sensitive adhesive layerhaving a dynamic modulus of 6×10⁶ dyne/cm² at 60° C.

EXAMPLE 7

A toluene solution of a pressure-sensitive adhesive prepared by adding0.01 part of a silicone oil (dimethylpolysiloxane; SH-200) to 90 partsof a polyisobutylene having a viscosity-average molecular weight ofabout 35,000 and a dynamic modulus of 3×10⁵ dyne/cm² at 60° C. and 10parts of a polyisobutylene having a viscosity-average molecular weightof 1,200,000 and a dynamic modulus of 3×10⁶ dyne/cm² at 60° C. wascoated on a 40 μm-thick film made of a polypropylene/polyethylene blend(weight ratio; 9/1), and then dried at 120° C. for 3 minutes, therebyobtaining a protective sheet having a pressure-sensitive adhesive layerwith a thickness of 10 μm.

EXAMPLE 8

A protective sheet was obtained in the same manner as in Example 7except that the amount of the silicone oil added was changed to 0.3part.

EXAMPLE 9

A protective sheet was obtained in the same manner as in Example 7except that a pressure-sensitive adhesive prepared by adding 0.1 part ofa silicone oil (alkyl-aralkyl-polyether-modified silicone oil; SF-8419)to 100 parts of a polyisobutylene having a viscosity-average molecularweight of 1,200,000 and a dynamic modulus of 3×10⁶ dyne/cm² at 60° C.was used.

EXAMPLE 10

A protective sheet was obtained in the same manner as in Example 9except that the amount of the silicone oil added was changed to 0.4part.

EXAMPLE 11

A protective sheet was obtained in the same manner as in Example 7except that a pressure-sensitive adhesive prepared by adding 0.2 part ofa silicone oil (alkyl-modified silicone oil; SH-230) to a rubber-basedpressure-sensitive adhesive comprising a mixture of 100 parts of KratonG-1657 and 40 parts of the hydrogenated petroleum resin and having adynamic modulus of 5×10⁶ dyne/cm² at 60° C. was used.

EXAMPLE 12

A toluene solution of a pressure-sensitive adhesive prepared by adding0.015 part of an acrylic polymer (Polyflow No. 9) having aweight-average molecular weight of 30,000 to 90 parts of apolyisobutylene having a viscosity-average molecular weight of about35,000 and a dynamic modulus of 3×10⁵ dyne/cm² at 60° C. and 10 parts ofa polyisobutylene having a viscosity-average molecular weight of1,200,000 and a dynamic modulus of 3×10⁶ dyne/cm² at 60° C. was coated,on a 40 μm-thick film made of a polypropylene/polyethylene blend (weightratio; 1/9), and then dried at 120° C. for 3 minutes, thereby obtaininga protective sheet having a pressure-sensitive adhesive layer with athickness of 10 μm.

EXAMPLE 13

A protective sheet was obtained in the same manner as in Example 12except that the amount of the acrylic polymer added was changed to 0.3part.

EXAMPLE 14

A protective sheet was obtained in the same manner as in Example 12except that a pressure-sensitive adhesive prepared by adding 0.2 part ofan acrylic polymer (Modaflow) having a weight-average molecular weightof 37,000 to 100 parts of a polyisobutylene having a viscosity-averagemolecular weight of 1,200,000 and a dynamic modulus of 3×10⁶ dyne/cm² at60° C. was used.

EXAMPLE 15

A protective sheet was obtained in the same manner as in Example 14except that the amount of the acrylic polymer added was changed to 1.5parts.

EXAMPLE 16

A protective sheet was obtained in the same manner as in Example 12except that a pressure-sensitive adhesive prepared by adding 0.8 part ofthe acrylic polymer (Modaflow) to a rubber-based pressure-sensitiveadhesive comprising a mixture of 100 parts of Kraton G-1657 and 40 partsof the hydrogenated petroleum resin and having a dynamic modulus of5×10⁶ dyne/cm² at 60° C. was used.

Comparative Example 1

A protective sheet was obtained in the same manner as in Example 1except that a rubber-based pressure-sensitive adhesive consisting ofonly a polyisobutylene having a viscosity-average molecular weight of35,000 was used to form a pressure-sensitive adhesive layer having adynamic modulus of 1×10⁵ dyne/cm² at 60° C.

Comparative Example 2

A protective sheet was obtained in the same manner as in Example 1except that a rubber-based pressure-sensitive adhesive consisting ofKraton G-1657 only was used to form a pressure-sensitive adhesive layerhaving a dynamic modulus of 9×10⁶ dyne/cm² at 60° C.

Evaluation Tests

Each of the protective sheets obtained in the Examples and ComparativeExamples was subjected to the following tests.

Adhesive remaining

The protective sheet was adhered at room temperature to a plate coveredwith an alkyd-melamine paint film having a glass transition point of 95°C. The resulting plate was placed in a chamber under high-temperatureconditions, taken out therefrom, and then allowed to stand at roomtemperature for 3 hours. Subsequently, the protective sheet was strippedoff the plate, and the plate was examined as to whether part of thepressure-sensitive adhesive had been remained on the plate surfaceespecially around the area corresponding to the edge parts of theprotective sheet.

In the above procedure, the high-temperature conditions were 80° C.×24hours.

Paint film deformation

The alkyd-melamine paint film after the adhesive remaining testdescribed above was visually examined as to whether a gap had beenformed in the paint film especially around the area corresponding to theedge parts of the protective sheet. Gaps having a depth below 0.1 μmwere regarded as none, while gaps having a depth of 0.1 μm or more wereregarded as present; the FIGURE illustrates the depth (d) of a gap 21.

Initial adhesion strength

The protective sheet was adhered to a plate covered with analkyd-melamine paint film having a glass transition point of 95° C. bymoving a 2 kg rubber roller forward and backward once on the protectivesheet at a temperature as specified below. The resulting plate wasallowed to stand for a period of time as specified below at the sametemperature, and the protective sheet was then peeled to measure theadhesion strength (180° peeling; peeling rate, 300 mm/min).

The temperature and the time for allowing the plate to stand were 23° C.and 30 minutes.

Adhesion strength after exposure (Strippability)

The protective sheet was adhered to a plate covered with analkyd-melamine paint film having a glass transition point of 95° C. bymoving a 2 kg rubber roller forward and backward once on the protectivesheet at a temperature as specified below. The resulting plate wasexposed under conditions as specified below, subsequently allowed tostand at 23° C. for 30 minutes, and then examined for adhesion strengthin the same manner as in the initial adhesion strength test above.

For Examples 1 to 6 and Comparative Examples 1 and 2, the temperaturewas 23° C. and the exposure was conducted by 100 hour irradiation in asunshine weatherometer. For the remainder examples, the temperature was23° C. and the exposure was conducted by 3 month outdoor weathering inOkinawa.

The results obtained are shown in the Table below.

                  TABLE 1    ______________________________________                     Adhesion strength (g/20 mm)    Adhesive    Paint film           After    remaining   deformation                           Initial   exposure    ______________________________________    Ex. 1 None      None       450     560    Ex. 2 None      None       560     620    Ex. 3 None      None       150     580    Ex. 4 None      None       200     580    Ex. 5 None      None       270     400    Ex. 6 None      None       230     350    Ex. 7 None      None       800     810    Ex. 8 None      None       400     420    Ex. 9 None      None       290     300    Ex. 10          None      None       150     180    Ex. 11          None      None       200     210    Ex. 12          None      None       750     790    Ex. 13          None      None       250     300    Ex. 14          None      None       220     240    Ex. 15          None      None       60      95    Ex. 16          None      None       130     150    Comp. Present   None       Cohesive                                       Measurement    Ex. 1                      failure impossible    Comp. None      Present    35      160    Ex. 2    ______________________________________

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A sheet for protecting a melamine-alkyd,melamine-acrylic or urethane automotive paint film wherein said sheetcomprises a plastic substrate having a thickness of 10 to 100 μm made ofa polypropylene and polyethylene blend and having formed on one sidethereof a layer of a rubber-based pressure-sensitive adhesive having asolubility parameter (SP) value of at least 1 larger or smaller than theautomotive paint film, a dynamic modulus of from 4×10⁵ to 7×10⁶ dyne/cm²at 60° C., an adhesion strength of 100 to 1,000 g/20 mm and a thicknessof 5 to 50 μm, said adhesive comprising an isobutylene-based polymerwhich is not curable, wherein said sheet has a ratio of (i) adhesionstrength after exposure at 23° C. for 100 hours in a sunshineweatherometer to (ii) initial adhesion strength of 1.01 to 3.86 whereinthe rubber-based pressure-sensitive adhesive contains 0.01 to 2 parts byweight of an acrylic polymer having a weight-average molecular weight of1,000 to 50,000 per 100 parts by weight of the rubber-basedpressure-sensitive adhesive.
 2. A sheet as claimed in claim 1, whereinthe acrylic polymer is obtained by solution polymerization of at leastone alkyl ester of acrylic acid or methacrylic acid having 1 to 18carbon atoms in the alkyl moiety.
 3. A sheet as claimed in claim 2,wherein the acrylic polymer is a liquid acrylic polymer having astructural unit represented by the formula ##STR3## wherein R¹ is ahydrogen atom or a methyl group, and R² is an alkyl group having 1 to 8carbon atoms.
 4. A sheet as claimed in claim 3, wherein the liquidacrylic polymer has a glass transition point of 0° C. or less.